The phony bomb was discovered at approximately 10:45 a.m. and the high school began student evacuations shortly after sending students to the campus track stadium to await bus transport home or to be picked up by their parents.
But about one hour later investigators gave an all clear. The high school was reopened for classes for the students still on campus and the adjacent middle school ended its lockdown.
Law enforcement who rushed to the school in response to the perceived threat also included the Ohio State Highway Patrol and Franklin Police Division.
The high school enrolls 1,741 students and 933 students attend Middletown Middle School.
None of Middletown’s eight other public schools were affected by the incident, said Elizabeth Beadle, spokeswoman for the 6,300-student district.
Beadle said: “We take things seriously. At the high school we have an amazing team that made an amazing response.”
The threat comes less than a week after hoax active shooter threats closed eight schools in Ohio, including Princeton High School in northern Hamilton County.
Staff Writer Lauren Pack contributed to this story.